Virtually all complex integrated circuits are designed with the use of computer aided design (CAD) tools. Some CAD tools, called simulators, help the circuit designer verify the operation of a proposed circuit. Another type of CAD tool, called a silicon compiler (also sometimes known as automatic layout or place and route systems), generates the semiconductor mask patterns from a detailed circuit specification. One task that must be performed by a silicon compiler is that it must route connections between the components of the circuit. Such components are often called cells.